1. Field of the Invention
This invention relates to controlling an engine using an ionization probe to detect cylinder combustion.
2. Prior Art
It is known to control various engine operating parameters, such as composition of exhaust emissions, in accordance with the relative time of occurrence of the combustion chamber peak pressure and piston top dead center during the combustion process. One of the known methods of detecting combustion chamber pressure is using an ionization probe placed in the combustion chamber.
For example, U.S. Pat. No. 4,304,203 issued to Garcea et al; U.S. Pat. No. 4,377,140 issued to Latsch and British Pat. Nos. 1,512,213 and 2,060,062A teach the use of an ionization sensing probe to control the combustion cycle in an internal combustion engine. Closed loop control using an ionic current sensor to determine the end of the ignition phase of a combustible mixture in an internal combustion engine can be used to adapt ignition timing to compensate for conditions such as thermal status of the engine, characteristics of the fuel and of the combustion air, engine wear and so on.
U.S. Pat. No. 4,345,154 issued to Bainbridge teaches an ionization sensing device to detect harmful gases in a gaseous medium. To compensate for erratic changes in the electrical output signals of the sensing cell caused by variations in the flow of gas, a bias voltage is impressed upon the sensing device.
To improve use of an ionization probe in connection with an engine control system, it is desirable to eliminate the cable capacitance effects of the cable coupling the ionization probe to the electronic engine control. The ionization probe usually includes a metallic probe that is inserted through the cylinder wall and is electrically insulated from the metal cylinder block. When combustion occurs in the cylinder, a number of free ions are created in the flame front. The free ions in the flame front cause a current to flow and result in a decrease in the effective resistance between the probe and the cylinder walls. Typical resistance changes are from open circuit, indicating no combustion, to one to two megohms during combustion. The probe is used to measure time of arrival of the flame front relative to the spark event. The time of arrival data is used to determine the quality of the burn cycle.
The probe has high impedance and the signal wire to the probe must be shielded to prevent noise pickup. This is particularly true in automotive applications where the engine control module must be remotely located from the sensor probe and there is the possibility of noise pickup from the ignition system.
It is known to use a relatively high impedance sensing resistor, for example one megohm, in series with the sensing probe (see FIG. 1). The drawback to this approach is the relatively slow time response that occurs as a result of the high impedance and the capacitance of the shielded coaxial cable lead.
For example, if the series resistance is one megohm and the capacitance of the coaxial cable is about 500 pico-farads, the time constant, due to the capacitance of the cable is about 500 microseconds. At 6000 rpm of engine operation, 500 microseconds is equivalent to 18.degree. of crank rotation. Advantageously, good engine control requires that the time of arrival of the flame front be measured to within at least plus or minus one degree of crank rotation. Accordingly, a technique which measures flame front arrival to within 18.degree. of crank rotation is not acceptable.
It would be desirable to keep the time response of the sensor and interface circuit to a much smaller amount and to reduce the capacitive effect of the cable.